Proton electrolyte membranes (PEMs) are components of fuel cells, hydrogen separation/purification, reforming/partial oxidation of hydrocarbon fuels, contaminant removal, gas sensing, and other processes relevant to energy storage and conversion. While various electrolyte membranes have been studied in many years, the existing membranes are still inadequate in performance for many applications.
The widely used perfluorosulfonic polymers (mainly Nafion®) have serious disadvantages, such as low proton conductivity over 100° C. due to loss of water, large amount of fuel crossover, dimensional changes with water contents, high cost, and the reduction of —SO3H groups under fuel cell working conditions.
These limitations have stimulated the development of many other proton conducting membranes, including polymer electrolytes with nanometer-sized hygroscopic metal oxides, sulfonated aromatic polymer membranes, polymer-H3PO4 membranes, and hybrid inorganic-organic copolymer membranes doped with proton-conductive components, including H3PO4, heteropolyacids, and —SO3H groups.
Among all above proton conducting membranes developed in recent years, polybenzimidazole (PBI)—H3PO4 membranes have the best performance. PBI—H3PO4 membranes have high proton conductivity above 150° C., good mechanical properties and high thermal stability (J. Electrochem. Soc. 1995, Vol. 142, p. L121). However, in PBI—H3PO4 membranes, H3PO4 can leach out easily from such pure organic polymer membranes, especially when H3PO4 content is high.
Meanwhile, when the content of H3PO4 is too high, the mechanical properties are degraded. Polyvinazene-H3PO4 was reported to have high proton conductivity from 150° C. to 200° C., but the decomposition of —CN groups under acidic conditions limits its application as the electrolyte material in fuel cells (Abstract of ECS meeting, Orlando, Fla., USA, October, 2003).
More recently, a polystyrene with imidazole terminated flexible side chains was synthesized. It was thermally stable up to 400° C., but the proton conductivity is too low to be used in PEM fuel cells (˜10−4 S/cm at 200° C., Electrochimica Acta, 48, 2165, 2003).
Accordingly, the development of novel electrolyte membranes with high proton conductivity in low humidity, dense structure, and good mechanical properties is still the key to the successful development of high temperature PEM fuel cells and other electrochemical devices.